In many cases, optical fibers are connected to each other by removing coverings of the optical fibers and fusion-splicing bare fibers of the optical fibers to each other. Such portions of the optical fibers where the coverings have been removed are vulnerable to external forces and may be broken when any impact or vibration is exerted to the optical fibers. Therefore, portions of the optical fibers where the coverings have been removed, such as a fusion splice portion, are received within a fiber accommodation portion having a high tensile strength and fixed by a resin or the like.
Furthermore, in a case of a polymer cladding fiber, an air cladding confines light at portions where a covering has been removed. If a foreign material is attached to the portions where the covering has been removed, then light in the optical fiber may leak into the attached foreign material. In a case of high-power light, the optical fiber may be burnt out. Therefore, there has been known an optical fiber protection structure in which a cover member is disposed above the aforementioned fiber accommodation portion to prevent any foreign material from being attached to a portion where the covering has been removed (see, e.g., Patent Literature 1).
FIG. 1 is a front view schematically showing such a conventional optical fiber protection structure 800, FIG. 2 is a cross-sectional view taken along line A-A of FIG. 1, and FIG. 3 is a cross-sectional view taken along line B-B of FIG. 2. As shown in FIGS. 1 and 2, the conventional optical fiber protection structure 800 has a fiber accommodation portion 820, in which a groove 810 is formed, and a cover member 830 disposed on the fiber accommodation portion 820. As shown in FIGS. 2 and 3, portions of two optical fibers 840 and 840 where the coverings have been removed and a fusion splice portion 860 are received in the groove 810 of the optical fiber protection structure 800. The optical fibers 840 and 840 are fixed to the fiber accommodation portion 820 at both ends of the groove 810 by resins 870. The cover member 830 is fixed onto the fiber accommodation portion 820. Each of the fiber accommodation portion 820 and the cover member 830 is formed of a material having a high tensile strength.
In such a conventional optical fiber protection structure 800, the rigid cover member 830 is placed on the resins 870 for fixing the optical fibers 840. Therefore, if the resins 870 expand under a high-temperature environment or a high-humidity environment, then the expansion of the resins 870 is inhibited by the cover member 830 placed on the resins 870. As a result, as indicated by arrows in FIG. 3, stresses are applied to the optical fibers 840 so that the optical fibers 840 are compressed. Thus, light propagating through the optical fibers 840 tends to be coupled to a higher mode so as to leak out of the optical fibers 840, so that the optical loss increases. For example, when high-power light from a fiber laser propagates through the optical fibers 840, such optical loss may increase the temperature of the optical fibers 840 and the optical fiber protection structure 800.